1. Field of the Invention
The invention relates to permeable membranes. More particularly, it relates to the preventing of the degradation of the permeation characteristics of permeable membranes employed in as separation operations.
2. Description of the Prior Art
Permeable membrane processes and systems are known in the art and have been employed or considered for a wide variety of gas and liquid separations. In such operations, a feed stream is brought into contact with the surface of a membrane, and the more readily permeable component of the feed stream is recovered as a permeate stream, with the less readily permeable component being withdrawn from the membrane system as a non-permeate stream.
The inherent simplicity of such fluid separation operations constitutes an incentive in the art to expand the use of membrane systems in practical commercial operations. In this regard, it will be appreciated that the selectivity and permeability characteristics of such membrane systems must be compatible with the overall production requirements of a given application. It is also necessary, of course, that the membranes exhibit acceptable stability and do not suffer undue degradation of their performance properties in the course of practical commercial operations.
Air separation applications constitute a highly desirable field of use for permeable membranes. In such applications, oxygen is typically the more readily permeable component of the feed air for particular membranes, and is withdrawn as the permeate gas. In such embodiments, nitrogen is the less readily permeable component, and is recovered as non-permeate gas. In air separation applications, it has been found that the performance characteristics of the membranes are sensitive to the presence of certain contaminants in the feed air stream. Exposure to such contaminants may result in a significant reduction in the permeability of the membrane in use. Fortunately, most contaminants commonly present in ambient air, such as light hydrocarbons, H.sub.2 O and CO.sub.2, have been found to result in, at most, a modest decrease in membrane permeability. The presence of even relatively low concentrations, e.g., less than 1 ppm by volume as C.sub.1, of heavy hydrocarbon oil vapors, such as might enter the feed air stream from an oil lubricated air compressor, can result in rapid and extensive loss of membrane permeability.
Among the major cost components of membrane systems for air or other gas separations are the membrane modules having a given membrane surface area and the air or other gas compressor unit to achieve a suitable permeation pressure level, so that the desired product gas quality and production can be achieved. The surface area requirements of the membrane system can generally be minimized for a given set of operating conditions, i.e., pressure, temperature and flow rate, by selecting a membrane material having a very high permeability for the more readily permeable component of the gas mixture being separated. Typically, for a given selectivity, the higher the membrane permeability, the less membrane area will be required for a given set of production requirements. Compressor costs are typically minimized, particularly in small plants, by the selection of oil lubricated rotary screw feed compressors.
During development work on membrane systems for air separation applications, it was found that the membrane permeability was subject to an initially rapid and significant decrease, followed by a further gradual decline over a period of months of operation. In response to such undesirable decrease in membrane permeability, it is presently common membrane practice to size the active membrane surface area with a safety factor sufficiently large to compensate for the anticipated permeability loss from all sources. Initially, therefore, the membrane system is significantly oversized for the desired product flow, and the feed gas compressor is typically operated in a turndown mode. As permeability degradation proceeds, either the operating temperature or pressure, or both, are increased to compensate for the decrease in permeability. In some instances, it is necessary or desirable to by-pass some of the modules in the membrane system initially, so as to reduce excess membrane area employed when the membranes exhibit their full permeability capability, and subsequently to bring such by-passed modules on stream as degradation of the initially employed modules progresses. In such instances, it will be appreciated that, in addition to a significant capital cost penalty associated with the provision of extra membrane surface area, such a membrane system must operate over a significant portion of its operating life under off design conditions, and that the control strategy for such a membrane system is more complex than for a system operating closer to its optimum design conditions.
As an alternative to such overdesign of membrane systems to compensate for degradation in use, attempts have been made to restore lost performance, but such efforts were initially unsuccessful in developing an economically feasible means for restoring the permeability of degraded membranes. While practical and economical means for restoring the permeability characteristics of degraded membranes may presently be available, obviating the need to discard degraded modules for replacement by new modules after an unduly short period of operation, further improvement in the response to the problem of membrane degradation is highly desirable in the membrane art. Neither overdesign of the membrane system nor interruption of gas product operations for membrane restoration treatment, or a combination of these approaches is an entirely satisfactory means for overcoming permeability degradation in practical commercial air or other gas separation operations.
It is an object of the invention, therefore, to provide an improved membrane system and process for overcoming the problem of degradation of permeability during gas production operations.
It is another object of the invention to provide a membrane system and process obviating the need for significant overdesign or for premature replacement of degraded membrane modules.
It is a further object of the invention to provide a membrane system and process for maintaining membrane permeability and minimizing the need for the interruption of gas producing operations for the treatment of membrane modules for restoration of the permeability characteristics thereof.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.